The present invention relates to a display device, and more particularly to a technique which is effectively applicable to an active-matrix type display device which forms a display drive circuit on a substrate surface thereof.
For example, an active-matrix type display device which uses thin film transistors (TFT) as active elements includes pixel regions on a liquid-crystal-side surface of one substrate out of a pair of substrates which are arranged to face each other with liquid crystal therebetween, wherein the pixel regions are surrounded by gate lines which extend in the x direction and are arranged in parallel in the y direction and drain lines which extend in the y direction and are arranged in parallel in the x direction.
The pixel region includes the thin film transistor (TFT) which is operated in response to the supply of a scanning signal from the gate line and a pixel electrode to which a video signal from the drain line is supplied by way of the thin film transistor.
The pixel electrode, for example, generates an electric field between the pixel electrode and a counter electrode which is formed on another substrate side so as to control the optical transmissivity of the liquid crystal between these electrodes with the electric field.
The liquid crystal display device includes a scanning signal drive circuit which supplies a scanning signal to respective gate signal lines and a video signal drive circuit which supplies a video signal to respective drain signal lines, wherein these drive circuits include shift registers.
There has been also known a polysilicon-type liquid crystal display device in which a semiconductor layer of the thin film transistor which constitutes the above-mentioned active element is made of polycrystalline silicon (polysilicon; p-Si).
In such a polysilicon-type liquid crystal display device, thin film transistors (for example, MIS transistors) which constitute the scanning signal drive circuit and the video signal drive circuit are also formed on a surface of the above-mentioned one substrate surface together with the thin film transistors constituting the thin film transistors using the same step.
There may be a case that polysilicon-type liquid crystal display device includes a high voltage for performing the inversion driving of the liquid crystal.
In general, the thin film transistor which uses the polysilicon as the material of the semiconductor layer forms a gate film by a deposition method and hence, a gate dielectric strength is basically low and is liable to be easily deteriorated by a penetration current whereby the thin film transistor is not suitable for the high-voltage application.
Although it is expected that the recrystallization technique for forming the polysilicon will be enhanced hereafter, there exists a possibility that the high voltage processing will become difficult along with the demand for higher performance.
On the other hand, in the field of the semiconductors, as a technique for enhancing the dielectric strength characteristics of the transistors, the LDD structure and the double gate structure have been known.